Enhancement of electrical conductivity in α-Al2O3 crystals doped with magnesium

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Abstract
Direct current and alternating current electrical measurements were performed to investigate the electrical conductivity of α-Al2O3:Mg samples with different concentrations of [Mg]0 centers (Mg ions each with a trapped hole) in the temperature interval 250–800 K. The concentration of [Mg]0 centers was monitored by the optical absorption peak at 2.56 eV. These centers were produced by oxidation at temperatures above 1050 K. The formation rate of [Mg]0 centers depends on the previous thermal history of the sample in either reducing or oxidizing atmosphere. At low electrical fields, dc measurements reveal blocking contacts. At high fields, the I–V characteristic is similar to that of a diode (corresponding to a blocking contact at one side of the sample and an ohmic contact at the other side) connected in series with the bulk resistance of the sample. Steady electroluminescence is emitted at the negative electrode when a current in excess of ≈10 μA passes through the sample, indicating that the majority of carriers are holes. Low voltage ac measurements show that the equivalent circuit for the sample is the bulk resistance in series with the junction capacitance (representing the blocking contacts) connected in parallel with a capacitance, which represents the dielectric constant of the sample. The values determined for the bulk resistance in both dc and ac experiments are in good agreement. The electrical conductivity of Al2O3:Mg crystals increases linearly with the concentration of [Mg]0 centers, regardless of the amount of other impurities also present in the crystals, and is four times higher in the c∥ than in the c⊥ direction. The conductivity is thermally activated with an activation energy of 0.68 eV, which is independent of: (1) the [Mg]0 content, (2) the crystallographic orientation, and (3) the concentration of other impurities. These results favor the small-polaron-motion mechanism.